This invention relates to spinal implants for use as anterior fixation devices, and to an implant which is to be placed into the intervertebral space left after the removal of a damaged spinal disc to assist in promoting interbody fusion.
A major cause of persistent, often disabling, back pain can arise by disruption of the disc annulus, chronic inflammation of the disc, or relative instability of the vertebral bodies surrounding a given disc, such as might occur due to a degenerative disease. In the more severe, disabling cases, some form of mechanical limitation to the movement of vertebrae on either side of the subject disc is necessary. In the more severe cases, the disc tissue is irreparably damaged, thereby necessitating removal of the entire disc. However, when the disc nucleus is removed without subsequent stabilization, the same disabling back pain often recurs due to persistent inflammation and/or instability.
Various approaches have been developed to stabilize the adjacent vertebral bodies following excision of the disc material. In one approach, two adjacent vertebra are fused posterio-laterally, according to a standard technique, by rigid plates engaged at their ends to respective vertebrae by way of screws. However, it has been found that posterior fusion with rigid plates can be associated with pseudoarthrosis and implant loosening and/or failure.
Another approach involves the use of posterio-lateral fusion and rigid posterior instrumentation, as described above, with the addition of an interbody graft or implant placed from the posterior approach. Although a significant improvement in fusion rate can be achieved with this approach, scarring of the spinal nerves can occur which may lead to pain or neurologic deficit.
In yet another approach, anterior grafts are implemented. Three general types of anterior devices are presently known. In one type, the entire vertebral body is replaced by an implant that spans the vertebral and two surrounding intradiscal spaces. This approach is frequently used where the vertebral body is also damaged by way of a tumor, a fracture and the like. In another approach, only the disc is replaced by a prosthetic disc. In this approach, the object is not fusion between the vertebrae at the instrumented level, but rather replacement of the disc mechanical features by an implant.
A third approach, to which the present invention is more particularly directed, concerns a device adapted to promote fusion or arthrodesis across the intradiscal space. In the upper lumbar spine, bone grafts can be placed within the disc and the surrounding vertebrae can be stabilized with a plate placed on the lateral vertebral body using screws projecting into the bodies. This approach is not possible in the lower lumbar disc spaces due to the regional vascular anatomy. Unfortunately, more than 90% of the lumbar fusions are performed in the lower lumbar region. Therefore, there exists a need for a device to stabilize adjacent vertebrae, such as in the lower lumbar region, which device is primarily confined within the given disc space.
While a few devices of this type are known (such as the device of Dove shown in U.S. Pat. No. 4,904,261), there remains a need for an anterior lumbar interbody fusion device that will improve the fusion rate between the adjacent vertebrae, while also preventing disc interspace collapse due to end plate cavitation, for instance. There is also a need for a fusion device that is safe and effective and still consistent with traditional surgical arthrodesis practice.
In connection with the development of the interbody, fusion device of the present invention, Applicants have ascertained certain design principals that should be met for a safe and effective device. One important principal is that the load transmitted between adjacent vertebrae should be on the strongest part of the vertebral body. One problem with some prior art fusion devices, and even some prosthetic disc devices, is that a large portion of the load is reacted against the weakest part of the vertebral body which can lead to cavitation of the device into the surrounding vertebral endplates with subsequent collapse of the intradiscal space and even damage to the vertebra itself.
Further, a preferred interbody fusion device should fit the patient's intradiscal anatomy and restore, as close as possible, the proper anatomic relationship of the disc, pedicle, nerve root and facet joint in order to avoid future physiological problems. Restoration of the normal disc height will also return the disc annulus to tension, reducing annular bulge or prolapse and promoting interspace stability. At the same time, the device should not shield the lumbar spine from all of the stresses normally borne by the spine, since it has been found that reduction of normal stress on the vertebrae can result in loss of bone mass and strength.
Finally, the interbody fusion device should provide enough interspace rigidity to eliminate the need for external fixation or rigid fixation plates. With these goals in mind, Applicants have developed the anterior interbody fusion device and method of the present invention.